Wireless digital communications systems are poised to offer a cost-effective alternative to cable and DSL data services. So called “WiMAX” technology, based on the IEEE 802.16e air interface standard is a promising framework for broadband wireless applications. It has the potential to enable full internet and digital voice services for both fixed and mobile users.
The physical layer architecture for IEEE 802.16e OFDMA systems is based on orthogonal frequency-division multiplexing (OFDM) modulation. Since OFDM divides the total bandwidth into multiple narrowband sub-bands, the effects of frequency selective fading are reduced. The OFDM system allows for a simple receiver structure while maintaining high link quality. The technology also employs adaptive modulation and coding in both the downlink and the uplink to deal with variations in link quality. This enables WiMAX to offer multiple date rates at the physical layer which can be adapted dynamically based on the integrity of the air link.
Multiple users share the total system bandwidth by multiplexing their data in both time and frequency. In an adaptive OFDM system, spectral efficiency can be improved by allocating time—frequency resources based on throughput requirements, quality of service constraints and the channel qualities of each user. A scheduler, which optimizes resource allocation for multiple active users, becomes a key element in such a solution. In present code-division multiple access (CDMA) systems, spectral efficiency decreases with an increasing number of active users because of intra-sector interference due to imperfect orthogonality of the downlinks. In an adaptive OFDM system, where orthogonal time—frequency resources are given to the user who can utilize them best, the spectral efficiency instead increases with the number of active users. This effect is known as multiuser diversity.
Multiuser diversity, in which sub-bands are allocated to users with the highest channel gains, can be quite effective for low-mobility users. However, multiuser diversity mode is prone to errors due to the bursty nature of interference. This is a tradeoff of concentrating user transmissions within sub-bands. When an interferer's burst is in the same sub-band as that of a user, the user may experience a low signal-to-interference ratio on a large fraction of the tones in its burst.
However, within as little as one frame transmission time, the propagation channel conditions of an interferer can change dramatically and/or some other, less-detrimental interferer may be scheduled within the user's sub-band. Therefore users may suddenly experience a very high signal-to-interference ratio on all or almost all tones in a burst. These unpredictable, large changes between high and low signal-to-interference ratios are undesirable.
Despite the advantages of multiuser diversity that come from choosing the sub-band with highest channel gain, throughput often cannot be improved as extra redundancy needs to be added to transmissions to compensate for worst case interference. The above mentioned variability of interfering users' propagation channels can be attributed to many aspects of practical wireless communications systems, e.g., mobility of users in the system, the application of different antenna array signal processing algorithms at the transmit and receive ends of both base stations and subscriber units, the difference in scheduling requirements for direct (base to subscriber) and reverse (subscriber to base) links, partially loaded sectors, and non-continuous transmissions in data-like services.
What is needed is a method to reduce the variance of interference between users while retaining the benefits of multiuser diversity. Such a solution would enable WiMAX and other OFDMA digital communications systems to achieve better resistance to potential interference problems such as those associated with mobile users operating under multiuser diversity.